1. Field of the Invention
The present invention relates to antenna, and more particularly, to a planar antenna.
2. Description of the Related Art
In general, an antenna is a special electric circuit used in connection with a high frequency circuit. A transmitting antenna efficiently converts the electric power of the high frequency circuit into wave energy and radiates the converted wave energy into free space. A receiving antenna efficiently converts the energy of an input wave into electric power and transmits it to the electric circuit. The antenna operates as an energy converter between the electric circuit wave and the radio wave. The size and shape of the antenna is appropriately designed so as to improve conversion efficiency.
The beam pattern of the antenna is important in determining the channel characteristic in a high speed radio communication system. FIG. 1 shows the beam pattern of an antenna provided for indoor high speed mobile radio communication. A base antenna 100 on a ceiling has a wide beam width 110. An antenna 130 attached to a user terminal 120 has a directional beam characteristic 140. Antennas for indoor high speed mobile communication use circular polarization in order to reduce the occurrence of a multipath fading phenomenon.
An antenna having the directional beam characteristic required for a receiving-end antenna can be easily realized using an array antenna. However, it is very difficult to realize a circularly polarized antenna having a wide beam angle such as that required for a base antenna. If a base antenna radiation pattern has a bowl shaped beam characteristic in which the antenna gain in the middle is low, the strength of the received electric field is uniform regardless of the position of a user. Therefore, it is possible to remarkably relax restrictions on the linear characteristics of RF transmitting and receiving ends, to easily realize an RF system, and considerably reduce manufacturing expenses.
In general, the planar antenna comprised of a dielectric and a conductor induces current to the surface of a conductor put on the dielectric or a slot and radiates electromagnetic wave energy into free space. The planar antenna occupies a small space since it can be attached to the surface of a terminal or a wall. It is possible to easily construct the array antenna using the planar antenna. Also, the manufacturing price of the planar antenna is low since it can be mass-produced. However, an undesired surface wave mode is generated other than a radiation mode since a dielectric layer is used. Accordingly, the efficiency of planar antenna is low. In the planar antenna, the wave is radiated into free space when current flows on the surface of the conductor and there exists a surface wave proceeding along the surface of the dielectric. The number of surface wave modes is proportional to the thickness of the dielectric layer. A minimum of one surface wave modes exists. The thickness of the dielectric layer should be reduced in order to suppress the number of surface wave modes. Only one mode (which cannot be removed) is generated when the thickness is reduced to no more than 1/4 of the radio wavelength in the dielectric. Accordingly, loss is minimized. In practice, however, since the wavelength is several mms in a millimetric wave bandwidth, the dielectric layer is so thin that it can be easily broken when it is manufactured.
FIG. 2A shows a micro-strip patch antenna which is widely used as a planar antenna. The micro-strip patch antenna is comprised of dielectric 20, a conductor 24 located under the dielectric 20, and a micro strip line 22 for feeding the current. FIG. 2B shows an example of a planar antenna using a multiple dielectric layer, which is comprised of the multiple dielectric layer 220, a conductor plate 210 positioned on the multiple dielectric layer including a ring slot 200, dielectric 240 positioned on the conductor plate 210, and a feeder unit 230 for feeding current to the ring slot 200.
In general, in the case of obtaining a circular polarization characteristic using the micro-strip patch antenna, it is very difficult to obtain an excellent axial ratio with respect to a wide angle. Also, the cross polarization characteristic is not good. Also, when the frequency is no less than the millimetric wave bandwidth, the planar antenna becomes so small that the dielectric is difficult to make and is easily broken by a slight shock.
A planar antenna formed by stacking various layers of dielectric having a thickness of 1/4 wavelength was once provided in order to make a thick and efficient planar antenna. In such a planar antenna, it is possible to increase the gain when the dielectric layers are stacked in an order in which the dielectric constants of the respective layers are high-low-high. However, it is not easy to make a multiple dielectric layer for a high millimetric wave bandwidth. That is because parasitic effects generated on the contact surface of different materials deteriorate the performance of the antenna when the antenna is not very precisely manufactured. Also, the performance may be affected if the antenna is twisted due to a change in temperature or compression.
It is possible to increase the gain by attaching an oval dielectric lens in the high millimetric wave bandwidth. However, the method is used in an extremely specialized field such as radio astronomy due to large expenses for precisely processing the lens and technological difficulties.
FIG. 3 shows a ring-slot antenna, which comprises a conductor plate 300, dielectric 310 under the conductor plate 300, and a slot 320 for radiating the radio wave. The ring-slot antenna is a uniplanar radiation device which replaces the micro-strip antenna in a millimetric wave frequency bandwidth. It can be easily manufactured even for a high frequency. The ring-slot antenna can employ various feeding methods such as a micro strip transmission line and a coplanar waveguide (CPW). It is possible to easily realize an antenna having a dual polarization characteristic with the ring-slot antenna. However, it is not easy to obtain the circular polarization characteristic at a wide angle though the above antenna is used. Since a ground surface exists on the same surface as the antenna, undesired backward radiation often occurs. A method of feeding to the ring-slot from two points with an angle difference of 90.degree. is used for realizing the dual polarization. In this case, the beam pattern is directional and asymmetrical. Also, it is difficult to obtain a desired axial ratio characteristic.